The New York Times/Siena poll showing President Biden losing to President Trump in five of six battleground states is prompting calls from prominent Democrats for Mr. Biden to step aside and let a new generation of Democratic leaders join the race. So which leading contenders are setting themselves up to step in should Mr. Biden bow out?

The clearest heir apparent is California’s governor Gavin Newsom. He just finished a presidential-like trip to China to talk climate change with President Xi Jinping. While 71 percent of Americans in battleground states think Mr. Biden is “too old to be an effective president,” Mr. Newsom got his photo op shaking hands with the leader of arguably America’s biggest adversary. His office also released beauty shots of the governor climbing the Great Wall of China — a sharp contrast to Mr. Biden’s stumbles.